Migrants often have lower mortality than natives in spite of relatively unfavorable social and economic characteristics. Although migrants have a short-run advantage due to the selective migration of healthy workers, persistent health and mortality differences between migrants and natives may be long-run effects of different experiences in childhood. We made use of a natural experiment resulting from rural-to-urban migration in the mid-19th century. Mortality was much higher in urban areas, especially in rapidly growing industrial cities. Migrants usually came from healthier rural origins as young adults. Data used in this study is available from 19th-century Belgian population registers describing two sites: a rapidly growing industrial city and a small town that became an industrial suburb. We found evidence of three processes that lead to differences between the mortality of migrants and natives. First, recent migrants had lower mortality than natives, because they were self-selected for good health when they arrived. This advantage decreased with time spent in the destination. Second, migrants from rural backgrounds had a disadvantage in epidemic years, because they had less experience with these diseases. Third, migrants from rural areas had lower mortality at older (but not younger) ages, even if they had migrated more than 10 years earlier. We interpret this as a long-run consequence of less exposure to disease in childhood.

Migrants often have lower mortality than natives in spite of relatively unfavorable social and economic characteristics. Although much of this difference can be attributed to the selective migration of healthy workers, some researchers have begun lo look for other factors that can explain persistent differences between the health of migrants and natives (Abraido-Lanz.a et al., 1999; Razum and Twardella, 2002). One possible explanation is that migrants and natives experienced conditions in their early lives with different implications for health and mortality in late adulthood and old age. Experiences in utero and in early childhood have been linked to a variety of general and specific conditions appearing many years later (Elo and Preston, 1992). In this article we compare migrants and natives in two 19th-century cities to look for long-term consequences of childhood environments. Before 1900 rural people who moved into cities were introduced to more than just the latest technologies and fashions. They also entered a more diverse and much more deadly epidemiological environment. Public health measures, water purification, and sanitary sewers did not bring urban death rates down to rural levels until the end of (he 19th century (Preston and van de Walle, 1978). If differences in exposure to disease in childhood had long-run consequences, migrants from rural areas should have carried this advantage with them when they moved to cities. The analysis in this paper is designed to separate this effect from other explanations of mortality differences between rural migrants and urban natives.

We have three reasons to expecl important differences in mortality between migrants and natives. First, migrants are a self-selected group who are usually healthy at the time they move (Williamson, 1988; Abraido-Lanza et al., 1999; Khlat and Darmon, 2003). Because most migrants are looking for more rewarding employment, people with poor health or disabilities are less likely to undertake the journey. This healthy-migrant hypothesis has been used to explain the lower mortality rates of migrants in North America and Europe. Because it depends upon selection at the individual level, migrants will be healthier than natives in the destination even if aggregate mortality and morbidity are higher in the origin than in the destination. Migrants in 19th-century industrial cities often had lower mortality than did urban natives (Alter et al., 1999), and we have observed that migrants who left a poor rural area often came from families with more surviving sons (Oris and Aller, 2001). A related mechanism lowering the mortality of migrants is called the salmon-bias effect. If migrants return to their origins when they become sick, their deaths will not be recorded in the destination area (Razum et al., 1998; Abraido-Lanza et al.).

Second, migrants may lack defenses against diseases that are more common at their destination than in their places of origin. Recent migrants are less likely to have either the biological immunities or social skills and social networks developed by natives to resist such diseases. This will result in high mortality shortly after arrival, but those who remain in the destination will acquire the “seasoning” that they need (Oris, 2001). Sharlin (1978) used this reasoning to argue that migrants in early modern cities had much higher mortality rates than urban natives. Smallpox, for example, was an epidemic disease in most of Europe, but it was endemic in some large cities. Because survivors of smallpox acquire permanent immunity, the disease disappears without a constant How of new victims. Smallpox epidemics in market towns tended to be 5 years apart, because it took that long to add enough vulnerable children to the population to sustain a new epidemic (Dobson, 1997). In large cities the disease never completely died out, and migrants from rural areas often contracted smallpox when they arrived in the city (Landers, 1987, p. 74).

Third, although migrants may lack immunities to specific diseases, there could be long-run advantages for those who suffered fewer diseases in childhood. Some childhood diseases, such as measles, can permanently compromise the immune system (Lunn, 1991). Others, such as typhoid fever, may be contracted in childhood but cause sickness and death at older ages (Benglsson and LindstrÃ¶m, 2(X)O; Trocsken, 2004). Diseases in early childhood may also compromise a child’s nutrition and development (Martorell and Habicht, 1986), which could have consequences much later. In addition, repeated episodes of disease may have cumulative effects that appear at older ages. Finch and Crimmins (2004) argued that episodes of sickness early in life result in inflammatory responses, which increase the likelihood of chronic diseases at older ages (see also Riley, 1989, p. 47, on “insult accumulation”). Thus, 19th-century migrants from less stressful environments may have been less prone to degenerative diseases, like heart diseases.

Each of these hypotheses suggests contrasts between migrants and natives that can be examined empirically. We expected the healthy-migrant effect would be strong immediately after migration but would diminish as time in the destination increased (Frisbie, Cho, and Hummer, 2001; Harding, 2003; Cho, Frisbie, and Rogers, 2004; Gray, Harding, and Reid, 2004;). Although current health is a good predictor of mortality in the near future, it is not a guarantee against sickness or death. This pattern is familiar from life and health insurance. Insurees are selected for good health, and they have lower mortality and morbidity rates immediately after the purchase of a policy. Current actuarial studies use quite long (e.g., 25 years) select periods (American Academy of Actuaries, 2002), but as recently as 1974 the Society of Actuaries for the United Kingdom concluded that a select period of 2 years was sufficient (Continuous Mortality Investigation Committee, 1974; Alter, 1983). The increasing duration of selection in insurance studies probably reflects improvements in diagnostic techniques and the shift toward deaths from chronic and degenerative diseases, which are easier to predict than infectious diseases, which predominated before the mid-20th century.

The seasoning hypothesis also implies that differences between migrants and natives should decrease over time, but it predicts higher rather than lower mortality among more recent migrants. In addition, migrants from places with less disease should face greater disadvantages in years with epidemic diseases, which may not have reached less populated areas. In our study area, nineteenth-century Eastern Belgium, the difference in life expectancy between rural areas and booming industrial towns sometimes exceeded 12 years (Neven, 1997). This implies that we should distinguish between migrants from rural areas with relatively low mortality and migrants from other urban areas, who should have had the same experiences as natives. In this respect the effect of seasoning would be different from the healthy-migrant effect, which should apply equally to all migrants regardless of origin.

The early-life-conditions hypothesis differs from the other two hypotheses in three respects. First, it should not be related to time spent in the destination. Second, migrants from rural areas should have an advantage but those from other urban areas should not. Again, we assumed that individuals raised in rural areas were exposed to fewer diseases and had fewer episodes of sickness as children. Third, early life conditions should be more strongly associated with mortality from chronic diseases than infectious diseases. This implies that we should see lower mortality at older ages among migrants from rural areas regardless of the lime that they spent in the destination.1

SOURCES AND METHODS

Data for this study was drawn from the population registers of Limbourg and Verviers, two cities in eastern Belgium. Verviers, which had developed into an important center for woolen textiles production during the 18th century, became the first location on the continent to develop mechanical spinning. The city grew rapidly in the first half of the 19th century, growing from about 10,000 inhabitants in 1800 to 24,053 in 1846, and 52,438 in 1899. This growth did nothing to improve public health, and life expectancy at birlh in Verviers was about 32 years in 1846 (Alter, 1988). Death rates remained high until the 1870s, when water from a new reservoir began to be distributed in the city.

Limbourg was a former stronghold of the Dukes of Limbourg, which retained an urban atmosphere in spite of its small size (Capron, 1996). Limbourg stagnated in the early 19th century as mechanized textile production in Verviers look lhe place of the artisanal methods used in the smaller city, but factories began to spread from Verviers to Limbourg around mid-century. Limbourg’s populalion fell from 1,945 in 1806 to 1,763 in 1846, but it grew to 4,627 in 1899. Expectation of life al birth in Limbourg was around 40 years at midcentury (Capron, 1998). This was belter than in Verviers, but siili 5 Io K) years below the life expectancies in surrounding rural areas. The gap between urban and rural areas was particularly large during cholera epidemics.

Limbourg and Verviers were chosen for study because they have long series of population registers thai can be supplemented by registers of births, deaths, and marriages. All municipalities in eastern Belgium were required to make lists of inhabitants in 1806, when the area was part of the Napoleonic Empire. Some places renewed these lists in the following decades. We obtained lists of inhabitants for Verviers in every year between 1 806 and 1846, except for 1807. Lists for Limbourg survive from 1808, 1829, 1837, and 1 84 1, and there are some lists of inmigrants during this period. In 1846 all municipalities in Belgium copied lhe census of that year into bound volumes, which were subsequently updated with births, deaths, marriages, and migration. Similar volumes were opened after censuses in 1856, 1866, 1880, 1890, and 1900, and this system eventually turned into Belgium’s current population register. Thus, we had individual-level information about population movements, including migralion, for almost the entire 19th century in both Limbourg and Verviers.

In this study we used data for individuals age 20 and older who were born before 1850. In Verviers we followed individuals, including migrants who entered the city, from 1850 until 1899. The data we used for Limbourg begins in 1850 and ends in 1889. The population registers of both communities reported places of birth for each individual. We also identified lhe dales that migrants entered Limbourg or Verviers. In most cases a date of entry was explicitly reported. When a date of entry was not available, we used the earliest dale that the individual was recorded in Limbourg or Verviers. Data from Limbourg and Verviers were pooled after experiments suggested thai results were similar in both communities.

We used the Cox (1972) semiparametric regression model to study factors associated wilh mortality. This model assumes thai hazard rales vary along some baseline lime dimension, which is not measured explicitly in the model. Covariates have proportional effects on the probability at all durations of lime. We used age as the baseline time dimension, which means lhat the regression models presented later implicitly controlled for age.

Table 1 presents distributions of lhe explanatory variables used in the analysis, and examples of complete models are in Table 2. Our focus was on the first two covariates, lime since arrival and origin, which we discuss in detail later. We controlled for city, gender, social class (occupation of head of household), year of birth, and year. As shown in Table 2, the relative risk of dying was 13% higher in Verviers than in Limbourg. Women faced risks that were 15% lower than for men, and mortality varied substantially by social class.

The years included in the analysis were divided into five groups: 1850 to 1865, 1866 to 1867, 1868 to 1882, 1883, and 1884 to 1899. This classification served two purposes. First, it was intended to delect trends in mortality. We noted earlier (hat mortality in Verviers began to decrease after water from a new dam was piped to the city. This development does not appear in the data presented here, except perhaps in the absence of cholera epidemics aller 1866. Second, we separated 1866 to 1867 and 1883 because they were years of unusually high mortality in the subpopulation studied here.” Crude death rates reached about 60 per 1,000 in both locations in 1866, a year in which cholera was widespread in Europe. The cause of high mortality in 1883 is less clear, however. Crude death rates were about 22 per 1,000 in 1883, but we show in Table 2 that the risk of dying was about 40% higher in our sample, which was older than the general population. City-wide crude death rates were actually higher in 1881 than in 1883, when smallpox was reported in Verviers. Mortality in 1881 was not particularly severe in the subpopulation studied here, however, and !hey had not been strongly affected by the smallpox epidemic in 1871 either. 3 We believe that typhoid fever was the main contributor to the peak in deaths in 1883, but it is also possible that influenza was present in that year.4

RESULTS

The first event history model in Table 2 showed a clear link between migration and health. The most recent migrants had the lowest risks of dying, but migrants’ advantage over natives decreased as time since their arrival increased. Figure 1 shows the effects of time since arrival on the relative risk of dying from the event history models in Table 2. All comparisons were to persons born in Limbourg or Verviers. The pattern in all years was quite clear, but there were important differences between years with and without epidemics. When epidemic years were excluded, the relationship between mortality and time since arrival became even more pronounced. Migrants who had arrived less than 5 years earlier were 31% less likely to die than natives, and those who had arrived 5 to 10 years earlier faced risks 25% below those of natives. As their time in Limbourg or Vervicrs increased, migrants became more like natives. The estimated risk for migrants who had 20 to 29 years was 7% below the risk of dying for natives, but it was not statistically significant. Migrants who had arrived more than 30 years in the past seem to have been more likely to die than natives. This is exactly the pattern that we expected the healthy-migrant effect to produce.

In contrast, migrants had no mortality advantage in years with epidemics. In epidemic years, there was no difference between the relative risks of dying of natives and of migrants who had arrived less than 20 years earlier. Although migrants were generally in better health than natives, they were not more resistant to specific infectious diseases: cholera in 1866 and typhoid fever in 1883. This suggests that a seasoning effect was also at work, and we next turn to comparisons among migrants to test this hypothesis.

As shown in Table 3 and Figure 2, we divided migrants by both time since arrival and origin. We split the first dimension into four groups: natives, migrants who arrived less than 10 years earlier, migrants who arrived more than 10 years earlier, and migrants who arrived before age 15. Migrants who arrived as children were in a separate category, because their experiences should be more similar to natives, as Kasakoff and Adams (2(K)O) found for 19th-century migrants in the United States. We used place of birth as an indicator of origin, which we divided into three groups: rural, urban, and other. Places of birth were classified on the basis of their population in 1900: Municipalities with less than 5,000 inhabitants in 1900 were considered rural, and places with more than 10,000 were considered urban. The other group consisted of places that fell between 5,(XX) and 10,000 in 1900 as well as places of birth outside of Belgium that we could not clearly identify as cities. Any categorization of this kind is bound to be arbitrary and imperfect. For example, some places in our rural group were actually industrial villages with close economic and epidemiological ties to neighboring cities. Furthermore, we did not have complete migration histories, and some people born in rural areas may have spent their childhood in other cities before arriving in Limbourg and Verviers. Because these problems would tend to reduce rather than increase the differences between categories, they should not affect the interpretation of comparisons between the rural and urban groups. Figure 2 focuses on this rural-urban comparison.

Figure 2 shows that the pattern in the previous figure was primarily due to migrants from rural areas. In nonepidemic years, migrants from rural areas had lower risks of dying than migrants from urban areas. During the two epidemic periods, rural migrants had the same or higher mortality than natives, whereas migrants from urban areas experienced mortality that was the same or lower than natives. A selection effect was also apparent among urban migrants who had been in Limbourg or Verviers less than 10 years, but it was not as strong as the effect for rural migrants with similar migration histories. Figure 2 tempts us to conclude that urban migrants had acquired greater immunity to epidemic diseases than natives of Limbourg and Verviers, but those results were not statistically significant. Overall, Figure 2 suggests that both the healthy-migrant and seasoning effects were present.

We next divided the sample by age and gender to examine lhe early-life-conditions hypothesis. Table 4 and Figure 3 show the comparison of the risks of death by rural versus urban origins among younger adults (ages 20 to 59) in nonepidemic years. In this age group, both female and male migrants born in rural areas had lower relative risks of dying than natives if ihey had arrived less than 10 years earlier. After 10 years of residence in Limbourg or Verviers, however, there was no difference in mortality between rural migrants and natives. Again, the results for migrants from urban areas lacked statistical significance, but it seems most likely that lhey had a smaller advantage than rural migrants.

Nonepidemic mortality for those age 60 and older is shown in Table 5 and Figure 4. At this age we found lower mortality only among recent male migrants born in rural areas, but we can explain the absence of a selection effect among female migrants in a way that is consistent with the healthy-migrant hypothesis. In a previous study, Alter (1999) found that older women who migrated into Verviers were often widows moving from rural areas to join the households of their married children. Older men, however, rarely joined their children’s households even if they were widowers (Neven, 2003). This implies thai migration of older women was noi selectively based on better health. Indeed, older female migrants may have been joining the households of lheir married children precisely because lheir health had worsened.5

Finally, migrants from rural areas who had arrived more than 10 years earlier also had lower mortality than did natives, and in this case the effect was statistically significant for women but not for men.6 We interpreted these results as evidence in favor of the early-life-conditions hypothesis. Migrants from urban areas had eilher higher (women) or slightly lower (men) mortality than natives, but the results lacked statistical significance.

In summary, we found that differences in mortality between migrants and natives were affected by lime since arrival, epidemics, and places of origin. The most consistent result was that recent migrants from rural areas had lower mortality than natives in years without epidemics. The only exception to lhis pattern was among older women migrating from rural areas, and other evidence suggests that their migration was more likely Io be due to poor rather than good health. The advantage of rural migrants disappeared during the two epidemics recorded in our data, however. Because our results for migrants born in urban areas were more diverse and none were statistically significant, it is more difficult to be certain about them. Nevertheless, urban-to-urban migrants appear to have had smaller advantages than rural migrants in nonepidemic years and to have fared as well or better than natives in epidemic years. We also saw evidence that at older ages migrants from rural areas may have retained an advantage over natives even when they had arrived more than 10 years earlier.

CONCLUSION

Differences in mortality between migrants and natives in 19th-eentury Limbourg and Verviers offer evidence in favor of all three hypotheses described earlier: selection, seasoning, and childhood environment. The most powerful of these mechanisms was selection. The risks of dying among recent migrants were as much as 30% lower than the risks faced by natives, but their advantage decreased as they spent more time at their destination. This implies that migrants were mostly in good health when they arrived. The main exception to this pattern provides further support for the selection hypothesis. Women from rural origins who migrated after the age of 60 had lhe same risk of dying as native women of the same age. Because these women were often widows joining the households of children who had migrated earlier, they were not selected for health in the same way as other migrants.

Although migrants had lower mortality than natives in normal years, they had no advantage when epidemic diseases were present. This can be interpreted in two ways. On one hand, it may simply mean that good health was not a defense against virulent infectious diseases, like cholera. On the other hand, migrants may have been al a disadvantage during epidemics, because they had experienced a smaller number of pathogens than urban natives, that is, they had less seasoning. Migrants also lacked social skills that urban natives had gained in previous epidemics. Access to healthy water was always a challenge in 19th-century towns (Goubert, 1987), and it became a question of life and death in periods of rapid urban growth (Oris, 1998). Our data suggest lhal migrants from urban origins, whose experiences would have been similar to urban natives, weathered epidemics better than migrants from rural backgrounds.

Finally, differences between migrants torn in rural and urban areas suggest longterm effects of childhood environments. Migrants from rural origins generally had lower mortality than those from urban origins, except when epidemic diseases were present. Although this advantage was short lived at younger adult ages, it was more persistent at older ages. Again the most interesting result is for female migrants from rural areas who were older than age 60. Those who had migrated more than 10 years earlier were almost 20% less likely to die than native women. Because the average time since arrival in this category is more than 25 years, most of these women had arrived in Limbourg or Verviers as young adults. The absence of this effect among younger migrants suggests that a rural childhood was more important for the degenerative diseases of old age than for the infectious diseases faced by young adults.

1 . Some reeenl studies of industrialized countries have also considered a fourth hypothesis Io explain persistent low mortality of immigrants (Abraido- Lanza et al., 1999; Razum and Twardella, 2002). The acculturation hypothesis suggests thai immigrants are protected from higher mortality by behaviors that they learned in their home countries. As immigrants and their children adopt the attitudes and behaviors of their new societies, like smoking and less healthy diets, their HH)HaIiIy becomes more like the average in their destination. This hypothesis is not appropriate for the population studied here. Most of the migrants in our sample moved short distances (less than 20 kilometers), and they joined urban societies that had been formed by earlier migrants from the same origins.

2. The cholera epidemic occurred in 1866. There were actually fewer deaths than normal in 1 867, because so many vulnerable people had died in the epidemic.

3. Smallpox probably had a weak impact in our sample, because they were older than the general population. Smallpox tends to strike younger people who lack immunities acquired from inoculation or previous contact with the disease.

5. Alter, Cliggett, and Urbiel (1996) found that old people living in the households of married children had higher mortality than those living alone or with unmarried children. They argued that older people in Verviers preferred to maintain separate households, and they gave up their independence only when they were physically unable to live on their own.

6. The ? value for women older than 60 was .052 for nonepidemic years, as shown in Table 5 and Figure 4. When all years were included, the p value was .011.

ALTER, G, L. CLIGGETT, and A. URBIEL, 1996. Household patterns of the elderly and the proximity of children in a 19th-century city, Verviers, Belgium, 1831-1846. p. 30-52, In T. Hareven (ed.) Aging and generational relations, de Gruyter. Berlin, Germany.

AMERICAN ACADEMY OE ACTUARIES, 2002. Final report of the American Academy of Actuaries’ Commissioners Standard Ordinary Task Force. Retrieved April 6. 2006 from http://www. actuary.org/liie/cso/reportjune02.pdf